1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and more particularly, to a technique for preventing a fracture caused by cracking and the deformation of a ceramic multilayer board in an ESD protection device that includes opposed discharge electrodes in a cavity of the ceramic multilayer board.
2. Description of the Related Art
ESD is a phenomenon in which a charged electroconductive body (for example, the human body) comes into contact with or comes into close proximity to another electroconductive body (for example, an electronic device) and discharges electricity. ESD causes damage or malfunctioning of electronic devices. To prevent ESD, it is necessary to protect circuits of the electronic devices from an excessively high discharge voltage. ESD protection devices, which are also known as surge absorbers, have been used.
An ESD protection device may be disposed between a signal line and ground. The ESD protection device includes a pair of opposed discharge electrodes and has a high resistance under normal operation. Thus, typically, a signal is not sent to the ground. An excessively high voltage generated by static electricity, for example, through an antenna of a mobile phone causes discharge between the discharge electrodes of the ESD protection device, which discharges the static electricity to the ground. Thus, the ESD device can protect circuits disposed downstream thereof from the static electricity.
An ESD protection device illustrated in an exploded perspective view of FIG. 13 and a cross-sectional view of FIG. 14 includes opposed discharge electrodes 6 in a cavity 5 of a ceramic multilayer board 7 made of insulating ceramic sheets 2. The discharge electrodes 6 are connected to external electrodes 1. The cavity 5 includes a discharge gas. Application of a breakdown voltage between the discharge electrodes 6 causes discharge between the discharge electrodes 6 in the cavity 5, discharging an excessively high voltage to the ground. Thus, the ESD protection device protects circuits disposed downstream thereof from the static electricity (see, for example, Japanese Unexamined Patent Application Publication No. 2001-43954).
However, such an ESD protection device has the following problems.
First, the discharge starting voltage depends primarily on the distance between discharge electrodes. However, the distance between the discharge electrodes may vary due to lot-to-lot variations or differences in shrinkage between a ceramic multilayer board and the discharge electrodes during a firing process. This produces variations in the discharge starting voltage of an ESD protection device. It is therefore difficult to precisely set the discharge starting voltage.
Second, the discharge electrodes disposed in a cavity may be detached from a ceramic multilayer board due to a reduced airtightness of the cavity or different thermal expansion coefficients between the substrate layers of the ceramic multilayer board and the discharge electrodes. This deteriorates the function of an ESD protection device, or alters the discharge starting voltage, which reduces the reliability of the ESD protection device.